The Pokébank has just been released in Pokémon Sun and Moon but apparently, players are starting to observe interesting bugs once they start transferring their Pokémon from previous Generations. A couple of bugs, in particular, happen when a player transfers his or her Pokémon from Red, Blue and Yellow into Sun and Moon. Here's what we know so far:
Bug In Transferring Pokémon from Red Blue and Yellow via Pokébank
For those that didn't know, the determining factor for Shiny status of a Pokémon is evaluated using their respective IVs, particularly Speed, Defense, Special must be at least 10 and Attack must be 2, 3, 6, 7,10, 11,14 or 15. The new Pokébank emulates this method but a glitch in the code somehow mess up the system which makes its readings on Attack and Defense IVs go backward. This means that Pokémon that would've been a shiny in GSC now lose their shiny status whilst other Pokémon that would not have been a shiny gains shiny status. A thread about this bug was detailed by ChezMere here.
Another bug that was observed was that Pokémon that learned moves exclusively for Gen 1 only will not be able to transfer into the current Generation. A clear example of this phenomenon is when a friend of yours has a Charizard with fly which it learned in Yellow. If that friend of yours traded with your copy in Red, this Pokémon will no longer be transferable in Sun and Moon.
Lastly, it appears that Gender is completely random when transferring Pokémon from Gen 1 to Gen 7. Unlike when transferring to Gen 2, where the formula is based on the Attack IV, the Pokébank isn't consistent with the one with GSC meaning, the gender of the Pokémon that would've been female when transferred to GSC won't necessarily be the same when transferred to Sun and Moon. It seems that the algorithm for determining shiny and gender status of Pokémon varies differently from Gen 2 and Gen 7. It's still unclear, however, if this is meant as intended or not.